chore: fix readme typo

[msmagnanijr]

In the readme, tls.cert is being used, but in the configuration file tls.crt is defined ( ssl_cert_file = '/opt/app-root/src/certs/tls.crt').

This simple mistake can easily cause someone to lose time if they don't pay attention.

[phracek]

Thanks for this pull request

[test]

[phracek]

Reference to configuration file https://github.com/sclorg/postgresql-container/blob/master/examples/enable-ssl/postgresql-cfg/ssl.conf#L3

[msmagnanijr]

Reference to configuration file https://github.com/sclorg/postgresql-container/blob/master/examples/enable-ssl/postgresql-cfg/ssl.conf#L3

Yep, As I said, it's a simple typo but it can waste someone's time. thanks

[zmiklank]

Thank you for the contribution!